Transistor-transistor logic (TTL) input stages typically require a positive supply voltage and use standard positive logic. The output of a TTL input stage must be low when its input is below 0.8 volts and high when its input is above 2.0 volts. This requirement must be met over the entire temperature range of operation. A specific example of TTL switching threshold voltages vs. temperature is shown in FIG. 1. The graph of FIG. 1 was obtained from an embodiment of the present invention. As illustrated by FIG. 1, a low output state is ensured over a range of temperatures where the input voltage is less than +0.8 volts, and a high output state is ensured over a range of temperatures when the input voltage is greater than +2.0 volts. Therefore, the high/low transition occurs somewhere between +0.8V and +2.0 V for the required range of temperatures.
A circuit which recognizes a high input level to be a logical 1 (true) and a low input level as a logical 0 (false) is said to use positive logic. TTL circuits which employ negative logic are also used.
TTL input stages typically require a positive supply voltage; however, some systems connected to the TTL output use only a negative supply voltage. Therefore, the requirement for a positive supply voltage to operate the TTL stage is cumbersome. Additionally, the TTL output signal has to be level shifted from a positive voltage to a negative voltage before it can control the system operating with a negative supply voltage. FIG. 2 (prior art) shows a typical TTL input stage 10 which requires a positive supply voltage and a level shifter 12 to control a system 14 using only a negative supply voltage.
What is needed is a TTL input stage for negative supply systems that does not require a positive supply.